Hearing protection with active noise damping is previously known in the art where there is disposed, interiorly in a protective muff, a microphone, a loudspeaker and a signal processing device, often of analog type. The protective muff has, in a traditional manner, a passive noise damping, which primarily damps out noise of higher frequencies, down to a region in the order of magnitude of 200 to 300 Hz. The signal processing device is formed in such a manner that the input signal from the microphone is phase reversed and its amplitude is adapted so that the signal emitted from the loudspeaker extinguishes the noise interiorly in the protective muff.
The analog signal processing device is rapid and consequently handles random, non-repeatable with good effect.
It is also previously known in the art to employ digital signal processing for noise damping. This signal processing is however slower than the analog processing, for which reason the digital signal processing is better suited for regularly recurring, repeatable noise such as engine noise, propeller noise and the like. Since the digital signal processing can be formed in such a manner that it seeks out the regularly recurring noise, extinguishing this noise can be made more effective than is possible with analog signal processing.
Combinations of analog and digital signal processing are also known in the art and are based on the use of the microphone interiorly in the protective muff.
That sound which impinges on the microphone disposed interiorly in the protective muff has passed through the passive noise damping, and also the analog and digital signal processing, for which reason the repeatable noise is damped and can be masked by the random, non-repeatable noise. This implies problems for the digital signal processing to trace and follow the repeatable noise, so that this can also be extinguished with good effect.
A general problem inherent in prior art technology is that the forming and fine-tuning as a rule takes place on the basis of some measurement norm or standard, e.g. A-weighted level, NRR (Noise Reduction Rating) etc. This implies that no, or slight, attention has been paid to ‘perceived residual spectrum’ interiorly in the hearing protection. Thus, low frequency noise has often been disregarded, and this noise may be extremely fatiguing and mask radio sound, i.e. communications, even though such sound does not give an indication on A-weighted measurements.